1. Field of the Invention
The invention relates to a method, apparatus and composition for wet etching, and more particularly, a method, apparatus and composition for wet etching layers formed on a semiconductor substrate
2. Description of Related Art
Various semiconductor fabrication techniques involve wet etching of layers grown on a semiconductor substrate. Selective etching is utilized to remove layers of one material without significantly degrading layers formed of another material.
For example, in component fabrication for micro-electro-mechanical systems (MEMS), it is conventional to form structural layers of polysilicon alternating with sacrificial layers of SiO2, whereafter the SiO2 layers are removed by wet etching with a composition that is selective for SiO2 and selective against polysilicon.
Wet etching compositions that are selective for polysilicon and selective against SiO2 are also known. Whereas HF etches SiO2, mixtures of HNO3 and HF will selectively etch polysilicon relative to SiO2, provided that the ratio of HNO3 to HF is sufficiently high.
However recent industry trends favor layers of silicon dioxide that are deposited at low temperature, and layers of polysilicon that are heavily doped. The usage of these modified materials, especially in combination, degrades the etch selectivity of conventional wet etchants and thus makes it very difficult to remove polycrystalline silicon selectively relative to silicon dioxide.
In particular, silicon oxide deposited at low temperature can show etch rates with mixtures of hydrofluoric acid and nitric acid up to four times greater than the etch rate of silicon oxides that are deposited at higher temperatures. Moreover, highly doped polycrystalline silicon can show significantly lower etch rates with the above shown mixtures. When these materials are used in combination the etch selectivity for polysilicon to silicon dioxide can drop from levels as high as 80 down to levels of as low as 2.